In a space vehicle, such as a communications satellite, it is often difficult to determine the precise attitude of the space vehicle. In a communications satellite, the need for precise attitude control enables the satellite to direct narrow beam communication signals to earth-based subscribers and control facilities. In other types of satellites, precise attitude control is useful in ensuring that solar panels which collect solar energy are constantly directed in the appropriate direction.
In a typical space vehicle, a device such as a magnetometer is used to determine the attitude of the vehicle based on changes in the geomagnetic field as the space vehicle travels in its trajectory. Because electronic systems on board the space vehicle can generate their own magnetic field, the field sensing elements of the magnetometer must be placed a suitable distance from the space vehicle electronics payload. Thus, the use of an attitude sensing magnetometer generally requires placement of the field sensing elements on a sizable boom in order to allow the accurate recording geomagnetic field information. The use of the magnetometer and the associated boom also increases the satellite mass.
A further drawback of a magnetometer is that the geomagnetic field must first be recorded prior to any actual attitude determination. During this recording process, the geomagnetic field is mapped and then correlated to the space vehicle's actual position. Thus, a secondary means for determining the space vehicle location must also be employed during this mapping process. This further complicates the use of a magnetometer for attitude sensing of a space vehicle. In addition, disturbances in the geomagnetic field caused by changes in sunspot activity can degrade the performance of the magnetometer.
Another method of determining the attitude of a space vehicle is through the use of a sun sensor. In a sun sensor, a beam of light from the sun enters through a small slit and illuminates a reticle unit. Based on the output of the reticle unit, the entry angle of the sunlight can be measured. In turn, the angle of the space vehicle can also be determined based on the reticle output.
A major drawback of the use of a sun sensor is that when the space vehicle is shadowed by the earth, no measurement can be made. Thus, no attitude determination can be made for long periods while the space vehicle is shadowed. Further, this approach can only provide two-axis information (roll and pitch), with rotation about the line of sight being unknown, requiring additional independent knowledge for resolution.
A third method of attitude determination makes use of a horizon sensor which typically senses the outlines of the earth or other body below the orbiting spacecraft using optical or infrared techniques. Geometrical algorithms are then used to derive two-axis information, such as roll and pitch attitude. A drawback to the horizon sensor approach is the potential for sun and moon interference which confuse the sensor. This approach also is limited to two-axis determination (roll and pitch), since no yaw information can be detected from outline sensing.
Therefore, what is needed are a method and system for accurate, continuous space vehicle attitude sensing using lightweight equipment that does not require prior magnetic field mapping in order to be effective in providing roll, pitch, and yaw information from a single measurement set of data.